1. Field of the Invention
The present invention relates to a membrane switch that is buried for use, e.g., below an automobile seat and operates such that its contacts are closed by person""s seating pressure.
2. Description of the Prior Art
Generally, in an existing membrane switch, a film-like first flexible insulating substrate at a part of which at least one first contact pattern is formed, and a film-like second flexible insulating substrate at a part of which at least one second contact pattern is formed are joined in facing relation via a thin spacer member so that corresponding first and second contact patterns are disposed in overlapped manner, wherein an opening is provided in each of areas in the thin spacer member in which the corresponding first and second contact patterns overlap one another, and each of the areas in which the corresponding first and second contact patterns overlap one another forms a contact of the switch. Wiring patterns respectively conductive-connected to the corresponding contact patterns are formed in one or both of the first flexible insulating substrate and the second flexible insulating substrate.
In the foregoing existing membrane switch, an area containing the formation portions of the contacts forms a switch body part, an area deriving from the switch body part forms a derivation part, and a tip area of the derivation part forms a terminal part (connection part) with connection terminals provided for connection with an external circuit. The contacts are conductive-connected with the connection terminals by the wiring patterns.
In this case, the existing membrane switch forms the first contact pattern, the second contact pattern, and the wiring patterns by printing them at required portions of the first flexible insulating substrate and the second flexible insulating substrate with a mixture of conductive silver powder and thermoplastic resin used as a binder material.
In the membrane switch having the foregoing structure, corresponding first and second contact patterns are usually out of contact; that is, each contact is usually open, and the first contact pattern and the second contact pattern are brought into conductive contact by pressure applied between the first contact pattern and the second contact pattern, so that the contact is closed.
This type of a membrane switch may be used in places where a change in the ambient temperature of the switch is relatively large. Particularly when the membrane switch is buried in an automobile seat so that it is used as a sensor element for judging whether an occupant uses the seat, if the periphery of the membrane switch is exposed to high temperatures and baggage is put on the seat of the parking automobile, pressure would remain applied to the membrane switch.
In the foregoing existing membrane switch, since the first contact pattern and the second contact pattern constituting the contacts are made of thermoplastic resin, if the ambient temperature of the membrane switch becomes high and pressure remains applied to the membrane switch, stress relaxation (creep) would occur at the contacts, reducing the actuating force to close the contacts. Specifically, there exists a problem in that prolonged stress applied to the membrane switch keeps the first contact pattern and the second contact pattern deformed and, as a result, the contacts are closed before normal (predetermined) pressure is applied to the contacts, disabling the correct operations of the sensor element.
The present invention has been made to solve such a problem and provides a membrane switch that, even if the ambient temperature of the switch becomes high and pressure is applied, is resistant to stress relaxation at the contacts and is capable of preventing the occurrence of contact malfunction.
To achieve the foregoing object, the present invention provides a membrane switch including the following means. A first flexible insulating substrate provided with a first contact pattern and a second flexible insulating substrate provided with a second contact pattern are disposed in facing relation via a spacer member, and wiring patterns are provided in at least one of the first flexible insulating substrate and the second flexible insulating substrate, wherein the wiring patterns are conductive layers containing resin made of a mixture of conductive powder and binder resin and at least one layer of the second contact pattern is a layer containing rigid resin that is more rigid than binder resin.
According to the foregoing means, since the wiring patterns are constituted by conductive layers containing resin made of a mixture of conductive powder and binder resin, in addition to the intrinsic flexibility of the first flexible insulating substrate and the second flexible insulating substrate, the entire membrane switch can be provided with satisfactory flexibility. Moreover, since a part of the second contact pattern is constituted by a layer containing more rigid resin than binder resin, even if the ambient temperature of the membrane switch becomes high during use, stress relaxation would occur at the contacts with less frequency and the heat resistance creep ability at the contacts could be increased.
In embodiments of the present invention, a membrane switch is constructed so that a first flexible insulating substrate at a part of which a first contact pattern is formed, and a second flexible insulating substrate at a part of which a second contact pattern is formed are joined in facing relation via a spacer member having an opening at an area in which the first contact pattern and the second contact pattern face each other; wiring patterns conductive-connected to corresponding contact patterns are formed on at least one of the first flexible insulating substrate and the second flexible insulating substrate; and the first contact pattern and the second contact pattern are conductive-connected by pressure applied to the formation portion of at least the second contact pattern, wherein the wiring patterns are conductive layers containing resin made of a mixture of conductive powder and binder resin, and at least one layer of the second contact pattern is a layer containing resin that is more rigid than the binder resin.
In a first concrete example of an embodiment of the present invention, in the membrane switch, binder resin to form the wiring patterns is thermoplastic resin and rigid resin to form the second contact pattern is thermosetting resin.
In a second concrete example of an embodiment of the present invention, in the membrane switch, the second contact pattern has a conductive layer at a layer below the layer containing thermosetting resin.
In a third concrete example of an embodiment of the present invention, in the membrane switch, the layer containing rigid resin comprises a conductive layer containing rigid resin formed by a mixture of conductive powder and binder resin that is more rigid than the binder resin of the wiring pattern.
In a fourth concrete example of an embodiment of the present invention, in the membrane switch, the wiring patterns are formed on the second flexible insulating substrate and the wiring patterns are constituted by two conductive layers, of which the lower uses silver powder as the conductive powder and thermoplastic resin as the binder resin, and the upper uses conductive carbon powder and thermoplastic resin as binder resin and covers the lower conductive layer; the layer containing rigid resin of the second contact pattern is constituted by a conductive layer containing rigid resin which contains a mixture of thermosetting resin as more rigid binder resin than the binder resin of the wiring patterns and carbon powder; and the lower conductive layer extends to the bottom (between the second flexible insulating substrate and the conductive layer containing rigid resin) of the conductive layer containing rigid resin.
In a fifth concrete example of an embodiment of the present invention, the membrane switch uses a polyester film as a base material of at least one of the first flexible insulating substrate and the second flexible insulating substrate; includes a switch body part containing the formation areas of the contact patterns and a derivation part protruding from the switch body part; and derivation patterns conductive-connected to the wiring patterns are formed in the derivation part and polyester resin is used as binder resin of the derivation patterns.
In a sixth concrete example of an embodiment of the present invention, in the membrane switch, the second flexible insulating substrate includes the switch body part, the derivation part, and a connection part for connecting the derivation part to an external circuit, and the connection part includes terminals using the rigid resin or rigid binder resin as a formation material.
In a seventh concrete example of an embodiment of the present invention, in the membrane switch, the second contact pattern, at the outermost layer, uses a conductive layer containing rigid resin made of phenol resin as rigid binder resin.
In an eighth concrete example of an embodiment of the present invention, in the membrane switch, the spacer member is film-like and is integrated with the first flexible insulating substrate and the second flexible insulating substrate by an adhesive, and has an opening which is smaller than the formation areas of the first contact pattern and the second contact pattern.
In a ninth concrete example of an embodiment of the present invention, in the membrane switch, the outermost layer of the terminals is a conductive layer containing rigid resin made of a mixture of conductive powder and more rigid binder resin than the binder resin of the wiring patterns.
In a tenth concrete example of an embodiment of the present invention, in the membrane switch, the first contact pattern includes a conductive layer containing rigid resin made of a mixture of conductive powder and more rigid binder resin than the binder resin of the wiring patterns and applies pressure to the both flexible insulating substrates.
According to the embodiments of the present invention, since the wiring patterns, like existing membrane switches, are constituted by conductive layers containing resin made of a mixture of conductive powder and binder resin, in addition to the intrinsic flexibility of the first flexible insulating substrate and the second flexible insulating substrate, like existing membrane switches, the entire membrane switch can be provided with satisfactory flexibility. Moreover, since apart of the second contact pattern is constituted by a layer containing more rigid resin than binder resin, preferably a layer containing rigid resin containing thermosetting resin, even if the ambient temperature of the membrane switch becomes high during use (when the portion in which the second contact pattern is formed is pressed), stress relaxation would less frequently occur at the contacts and the occurrence of malfunction such as the contacts that are closed when a predetermined amount of pressure is not applied could be prevented.
Of the embodiments of the present invention, according to the embodiment that a conductive layer is provided at a layer below the conductive layer containing rigid resin to constitute the second contact pattern, with the conductivity of the second contact pattern maintained, contacts resistant to high temperatures can be formed.
As a layer containing rigid resin to constitute the second contact pattern, an additional insulating layer can be provided. However, in the embodiments of the present invention, if a silver layer containing a mixture of binder resin made of thermoplastic resin and silver powder is formed as the base of the wiring patterns and the second contact pattern, and the silver layer in the wiring pattern parts is covered with an upper conductive layer containing a mixture of binder resin made of thermoplastic resin and carbon powder, the conduction resistance of the wiring patterns and the second contact pattern can be reduced and there is no need to worry about the corrosion of the silver layer. Furthermore, the two types of carbon layers provide resistance to stress relaxation at the contacts without impairing the flexibility of the membrane switch.
Of the embodiments of the present invention, according to the embodiment that a polyester film is used as the base material of at least one of the first flexible insulating substrate and the second flexible insulating substrate and polyester resin is used as the binder resin of the wiring patterns (the derivation patterns), the wiring patterns (the derivation patterns) are brought into more intimate contact with the flexible insulating substrates, and even if the derivation part is used in a relaxed manner, the wiring patterns would not disjoin from the flexible insulating substrates.
Of the embodiments of the present invention, according to the embodiment that a film material is used as the spacer member and the opening of the spacer member corresponding to a contact is formed to be smaller than the formation areas of the first contact pattern and the second contact pattern, even if the first contact pattern and the second contact pattern are formed using a printing means so as to cause a wide range of film thickness variation, the gap between the contacts could be decided by the board thickness of the spacer member used as a film material, the variation of the gap would become small, and pressure for closing the contacts could be kept almost constant.
Of the embodiments of the present invention, according to the embodiment that more rigid binder resin than the binder resin forming the wiring patterns is used as the binder resin forming the terminals of the connection part, the terminals of the connection part would be little shaved by repeated insertion and extraction operations between the connection part and the connector, and trouble due to conductive clippings can be avoided.
Furthermore, of the embodiments of the present invention, according to the embodiment that rigid phenol resin is used as binder resin at the outermost layer of the second contact pattern, highly wear-resistant, long-life contact parts can be formed.
Also, of the embodiments of the present invention, according to the embodiment that the first contact pattern includes a conductive layer containing rigid resin, even if pressure is applied from any of the two flexible insulating substrates, stress relaxation occurs less frequently and the heat resistance creep ability can be increased.